Multiphase Ceramics Research Articles

Systematic approach to preparing ceramic–glass composites with high translucency for dental restorations

ObjectiveCeramic composites are promising materials for dental restorations. However, it is difficult to prepare highly translucent composites due to the light scattering that occurs in multiphase ceramics. The objective of this work was to verify the effectiveness of a systematic approach in designing specific glass compositions with target properties in order to prepare glass infiltrated ceramic composites with high translucency. MethodsFirst it was necessary to calculate from literature data the viscosity of glass at the infiltration temperature using the SciGlass software. Then, a glass composition was designed for targeted viscosity and refractive index. The glass of the system SiO2–B2O3–Al2O3–La2O3–TiO2 prepared by melting the oxide raw materials was spontaneously infiltrated into porous alumina preforms at 1200°C. The optical properties were evaluated using a refractometer and a spectrophotometer. The absorption and scattering coefficients were calculated using the Kubelka–Munk model. ResultsThe light transmittance of prepared composite was significantly higher than a commercial ceramic–glass composite, due to the matching of glass and preform refractive indexes which decreased the scattering, and also to the decrease in absorption coefficient. SignificanceThe proposed systematic approach was efficient for development of glass infiltrated ceramic composites with high translucency, which benefits include the better aesthetic performance of the final prosthesis.

Oxidation Behavior Research of the SiC/B<sub>4</sub>C Multiphase Ceramics

Inthis investigation, the rare-earth oxide Yb2O3 combined with Al2O3served as sintering additives and SiC and B4C powder were applied to fabricate SiC/B4C multiphase ceramics composites by pressureless sintering. The results proved thatcombination of Al2O3 and Yb2O3 sinter additives were effective fordensification of SiC/B4C composites. The influence of oxidation time onthe phase constitution, micro-structure and oxidation behavior of SiC/B4C composites was investigated.Theformation of eutectic phase Yb2Si2O7 phase waswrapped on the SiC surface and it reduced further oxidation of SiC particles.The oxidation kineticcurves followed a parabolic rule.

Influence of Reinforcing Phase on the Oxidation Model of ZrB<sub>2</sub> Based Multi-Phase Ceramics

ZrB2, YAG and Al2O3 are widely applied because of some excellent performances, but ZrB2 is easily oxidized in the high-temperature air. To make the ZrB2 ceramics obtain better oxidation resistance, high-density ZrB2-YAG-Al2O3 ceramics were prepared. The oxidation mechanism of ZrB2-YAG-Al2O3 ceramics is investigated. When YAG-ZrB2 ceramics are exposed to air at temperatures from 1100°C¡æ to 1600°C¡æ, the weight change rate of YAG-ZrB2 ceramics is fleetly increased, the reacted speed of ZrB2 and O to form B2O3 and ZrO2 is more faster. The quantity of melted B2O3 is increased and carried to the surface, B2O3 is reacted with Al2O3 to form Al18B4O33. Al18B4O33 is melted and coated on the surface of ceramics to form protection layer for the oxidation resistance of ceramics at high temperature, which shows the similar with the influence of reinforcing phase on the oxidation model of ZrB2 based multi-phase ceramics.

Effects of Mg or Sr Doping on the Intrinsic Characteristics and Absorption Properties of Micro-nano BaFe12O19 Hollow Multiphase Ceramic Microspheres

Abstract Micro-nano BaM hollow multiphase ceramic microspheres (HMCMs) and Mg-doped and Sr-doped BaM HMCMs were prepared by combining the low-temperature self-reactive quenching method with heat treatment. The effects of Mg or Sr doping on the intrinsic characteristics and absorption properties of micro-nano BaM HMCMs were investigated. The results showed that Mg or Sr doping modified the phase compositions and particle size distributions of the HMCMs. The Mg- and Sr-doped BaM HMCMs had large real and imaginary permittivities and large real permeabilities at low frequencies relative to the, undoped HMCM. To maximize the absorption properties of the undoped and Sr- and Mg-doped HMCMs, optimal thicknesses of 8, 10 and 10 mm were used with corresponding reflectivities of −8.2, −31.5 and −26.6 dB, respectively.

Effect of B<sub>4</sub>C Content on Densification Behavior of SiC-B<sub>4</sub>C Ceramics

The SiC-B4C multi-phase ceramics was fabricated by gas-pressure sintering technology. The rare-earth oxide Al2O3combined with Er2O3/SiO2was served as sintering aids. The results were shown that the combination of Al2O3/Er2O3/SiO2sintering additives were effective for densification of SiC-B4C multi-phase ceramics. The influence of B4C content on the phase constitution, microstructure and densification behavior of the SiC-B4C multi-phase ceramics were detailed. The lose weight and volume shrinkage rate of SiC-B4C multi-phase ceramics had similar evolvement trend when B4C content increased. Keywords: Gas-Pressure Sintering, SiC-B4C multi-phase ceramics, densification behavior.

Melt processed multiphase ceramic waste forms for nuclear waste immobilization

Ceramic waste forms are promising hosts for nuclear waste immobilization as they have the potential for increased durability and waste loading compared with conventional borosilicate glass waste forms. Ceramics are generally processed using hot pressing, spark plasma sintering, and conventional solid-state reaction, however such methods can be prohibitively expensive or impractical at production scales. Recently, melt processing has been investigated as an alternative to solid-state sintering methods. Given that melter technology is currently in use for High Level Waste (HLW) vitrification in several countries, the technology readiness of melt processing appears to be advantageous over sintering methods. This work reports the development of candidate multi-phase ceramic compositions processed from a melt. Cr additions, developed to promote the formation and stability of a Cs containing hollandite phase were successfully incorporated into melt processed multi-phase ceramics. Control of the reduction–oxidation (Redox) conditions suppressed undesirable Cs–Mo containing phases, and additions of Al and Fe reduced the melting temperature.

Solidification Behavior of Lined Al<sub>2</sub>O<sub>3</sub>-ZrO<sub>2</sub> Multiphase Ceramics in SHS Composite Pipes

Hypoeutectic and hypereutectic Al2O3+ZrO2 multiphase ceramic-lined composite pipes were produced by using the gravitational separation self-propagate high-temperature (SHS) process. The microstructure of the ceramics was observed by means of SEM and EPMA. The multi-phase ceramics base consists of lamellar or rod-like eutecticum of ZrO2with Al2O3, and Al2O3dendrite is distributed between (Al2O3+ZrO2) eutecticum and the ZrO2is distributed on boundary area between (Al2O3+ZrO2) eutecticum in appearance of band and particle alone in the hypoeutectic multi-phase ceramics, and ZrO2is distributed between (Al2O3+ZrO2) eutecticum in appearance of snowflake-like or fishbone-like in hypereutectic multi-phase ceramics. On the basis of combustion synthesis, material thermodynamics, metallurgy dynamics and ceramics materials theory, the formation of microstructure mechanism have been systematically investigated.

Acid-Washing Behavior of TiB<sub>2</sub>/MgAl<sub>2</sub>O<sub>4</sub>/MgO Composite Powders

Using TiO2B2O3 powders as oxidizer,AlMg powders as reducer,the TiB2 multiphase ceramic are preparated with SHS, and analysed with SEM and XRD.The effect of HCl, H3PO4 to pickling of TiB2-based multiphase ceramics is explored by SEM and XRD analysis. Research suggests that: the main ingredients of TiB2-based multiphase ceramics is TiB2, MgO and MgAl2O4 three phases, few of reaction residual objects and impurities also exists; the multiphase ceramics is forming by many of hexagonal crystal type products and small particles reunion, hexagonal crystal type product is TiB2, small particles may be MgO, MgAl2O4 and other impurities product; MgO can be effectively washed by HCl and H3PO4 , but TiB2 and MgAl2O4 invalid. At room temperature, 30% HCl pickling has the best effect, the concentration of TiB2(ω) is up to 59.3%; the higher of the concentration, the better of acid-effect; In the pickling process, as the temperature rises, the concentration of TiB2 grows, but the temperature should not be too high. When temperature reach 75 °C, HCl volatilizes too quickly and inefficiently. At room temperature, 20% H3PO4 has the best pickling effect, the concentration of TiB2(ω) is up to 52.6%, but the best pickling effect is not well with concentration increasing; as the temperature increasing, purity of TiB2 is also increased; At 70°C, the concentration of TiB2(ω) after H3PO4 pickling is up to 61.9%, refine effect is good.

An Embedded Microstructural Model for Predicting the Strength and Toughness of Multi-phase Ceramics

The effect of microstructure on the strength and toughness of advanced ceramics has been investigated. The current work focuses on numerically modelling statistically representative microstructures and their implementation in multi-scale models. Finite volume based stress analysis is carried out on these microstructures to investigate the effect of individual material parameters on the strength and toughness of the bulk material. From the finite volume analysis it is found that the underlying microstructure affects the local stress and strain distributions within the numerical models. This in turn impacts on the strength and toughness of the bulk material model.

Nickel silicide nanocrystal-containing magnetoceramics from the bulk pyrolysis of polysilazane and nickelocene

Nickel silicide nanocrystal containing Si–Ni–C–N multiphase ceramics were synthesized by bulk pyrolysis of the mixture of polysilazane (PSZ) and nickelocene (NiCp2) at high temperature. The pyrolysis behavior of the precursors was investigated by TGA/DSC-Mass Spectrometry. The yield of the ceramic containing NiCp2 precursor was increased by 8–10% compared to pure PSZ. Nickel silicide nanocrystal containing ceramics were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. The NiCp2 in polymeric precursors induced the formation of α-Si3N4, SiC, graphitic carbon, and Ni2Si at 1100°C. Nickel silicide nanocrystals containing ceramics exhibited soft magnetism with ultralow hysteresis loss offering a convenient cost effective approach for the preparation of tunable transition metal-containing ceramics using a commercial inexpensive polymeric precursor of NiCp2.

<i>In Situ</i> Synthesis of TiB<sub>2</sub>-TiC<sub>0.8</sub>-40vol%SiC by Hot Pressing

TiB2-TiC0.8-40vol%SiC multiphase ceramics were prepared by in-situ hotpressing sintering. The phase composition and microstructures of the materials were characterized by optical mic- oscope, X-ray diffraction and scanning electron microscopy. The effects of sintering temperature on the phases, microstructures and mechanical properties of the ceramics were investigated. The results show that density, bending strength and fracture toughness of the ceramics are increased with the elevation of sintering temperature (1800-1950°C). High densified TiB2-TiC0.8-40vol%SiC multipha- se ceramics and optimized microstructure is obtained by sintering at 1900°C, in which the uniform distribution of lath-shape TiB2 and bulk TiC0.8 grains can be observed obviously. Nano-SiC particles distributed dispersively in the TiB2 and TiC0.8 grains and at boundaries. The Vickers hardness, fract- ure toughness, flexural strength and electrical conductivity of the TiB2-TiC0.8-40vol%SiC multipha- se ceramics sintered at 1900°C are 24.055GPa, 8.27±1.0MPa∙m1/2, 516.69MPa and 2.2×106S∙m-1, respectively. However, up to 1950°C, TiB2 and TiC0.8 grains gradually grew up, the bending stren- gth of multiphase ceramics was decreased greatly. In addition, TiB2, TiC0.8 and SiC particles were incorporated together to improve the particulate strength and toughness of composite material by the synergistic mechanism effects among the crystal phases in the multiphase ceramics, such as crack deflection, grain’s pull-out and fine-grain toughening.

The Classification Mechanism of ZrB<sub>2</sub> Powder Materials via the Sedimentation Method

With the development of science and technology, the more and more demand of ultra-fine powder is showed. The ZrB2/A1(OH)3-Y(OH)3 core-shell composite particles already are synthesized. To obtain the better A1(OH)3-Y(OH)3 composite shell, the higher coating ratio on the ZrB2 particles surfaces to prepare high performance multiphase ceramics, the classification of ZrB2 raw material is investigated by the sedimentation method. The sedimentation time of classing ZrB2 particles is decided for 30 minutes to reach fine ZrB2 particles with smaller size and narrower distribution is obtained.

Sintering Character of Coated Al<sub>2</sub>O<sub>3</sub>-Y<sub>2</sub>O<sub>3</sub>/ZrB<sub>2</sub> Composite Powder Materials via Spark Plasma Sintering

ZrB2 has some excellent physical performance and chemical stability, it has been widely applied in a lot of fields. In order to improve disadvantages of ZrB2 that the sintering densification of ZrB2 is too difficult, and it is easy oxidized at high temperature, in this paper, the sintering character of coated A12O3-Y2O3/ZrB2 composite powder materials via spark plasma sintering were investigated. Al2O3-Y2O3/ZrB2 composite powders were prepared by a co-precipitation methods. When the pH is 9, the encapsulted structure of A1(OH)3-Y(OH)3/ZrB2 composite powders is the best. By analyzing the ZrB2 surface status with TEM, the A12O3-Y2O3/ZrB2 composite powders were prepared under the calcining conditions of 600°C¡æ in argon. The high density ZrB2-YAG multi-phase ceramics are prepared via spark splama sintering, which indicate the raw materials adding A12O3-Y2O3 help for the densification of ZrB2 ceramics.

Combustion Synthesis of TiB<sub>2</sub>-TiC Ceramic

TiB2-TiC multiphase ceramics were prepared through altering the molar ratio of Mg and C and leached with suitable acid by the combustion synthesis reaction of Mg, C, TiO2 and B2O3. The samples were investigated by XRD and SEM, and the results showed the diameter of the TiB2 was 1.5μm and the shape of the TiC was irregular spherical particles. The diameter and the proportion of the TiB2 were increased with the increase of Mg powder. The formation mechanism of TiB2 has been studied. The optimized proportioning of the molar ratio of Mg and C was 3:2.

Preparation of Mullite-Aluminum Titanate-Cordierite Multiphase Ceramics

Mullite-aluminum titanate-cordierite multiphase ceramics were prepared by high alumina clinker, Aluminum Titanate and Cordierite. The sintering property and thermal shock resistance of composite materials were tested. The experimental results show that the sinter property and themal shock resistance of Mullite-aluminum titanate-cordierite multiphase ceramics are relatively preferably, which the materials composition are 30 wt.% high alumina clinker, 60 wt.% cordierite and 10 wt.% aluminum titanate. The component samples show porosity of 33.17%, volume density 1.9 % and normal temperature flexural strength 20.66 MPa. Thermal residual flexural strength of the samples is still as high as 10.29 Mpa by 5 times thermal residual tests, and there are only little flexural strength lower after three times earthquake test.

Experimental development of patch perthite from synthetic cryptoperthite: Microstructural evolution and chemical re-equilibration

In this study, deuteric coarsening of a lamellar cryptoperthite to a patch perthite has been experimentally induced for the first time. An homogeneous alkali feldspar, bulk composition Ab60Or40, was produced by molten salt ion-exchange with a gem quality orthoclase from Madagascar. This was then annealed isothermally for 32 days at 550 °C. This resulted in a coherent lamellar cryptoperthite (periodicity 28 nm) probably by spinodal decomposition and subsequent coarsening. The cryptoperthite was then reacted with 20 wt% H 2 O or 0.1 M HCl at 400 and 500 °C at 200, 400, and 1000 MPa for 96 h and for 192 h at 1500 MPa. At 1000 and 1500 MPa and 500 °C, the cryptoperthitic crystal fragments were partially replaced by a mosaic of albite and sanidine subgrains in the form of a patch perthite. Other than the Gibb’s free energy of reaction as derived from the chemical potentials of the reactant and product phases, the second principle mechanism responsible for driving this reaction is interface-coupled dissolution-reprecipitation driven by a reduction in the coherency strain energy stored within the initial cryptoperthite. The main indicator of this is a change in the microtexture due to the destruction of the coherent lamellar cryptoperthite and its replacement by an assemblage of incoherent subgrains. Detailed mass balancing, based on XRD and EPMA data, indicate that conversion of cryptoperthite to patch perthite is not isochemical. Experimental formation of patch perthite appears to take place in two steps. Between the original cryptoperthite and the patch perthite, TEM investigation identified a “transition zone,” characterized by a significant coarsening of the preexisting lamellar microstructure, without any sign of a loss in lattice coherency. This zone is probably the result of hydrous species diffusing into the cryptoperthite causing the H 2 O-enhanced diffusion of Na and K and hence lamellar coarsening. In the second step, this coarsened microstructure is replaced by the patch perthite. The polycrystalline patch perthite is characterized by a very complex distribution of albite and sanidine sub-grains. Even the larger albite or sanidine patches are polycrystalline. This inevitably results in a three-dimensional network of grain boundaries characterized by high defect densities resulting from the structural misfit between the albite and sanidine sub-grains. An increase in average grain size with distance from the interface indicates that secondary coarsening of the initially, very fine-grained intergrowth occurred. This was driven by the reduction of interface and surface energy similar to the coarsening observed in multiphase ceramics. In addition, the newly formed patch perthite is highly porous despite the fact that the results obtained in this study show that the replacement of cryptoperthite by patch perthite is nearly isovolumetric (ΔV ∼ -0.6%). The observed reaction-induced porosity results from differences in relative solubilities between the cryptoperthite and the patch perthite. Porosity abundance has been estimated at approximately 10 to 11 vol% based on the amount of quartz formed during the experiments. Three-dimensional investigations via FIB serial sectioning indicate almost no interconnectivity between pores. A consequence of albite and sanidine sub-grain coarsening is the constant redistribution of the porosity and the sub-grain boundaries within the patch perthite mosaic over time. This suggests that a dynamically evolving porosity could provide an important transport mechanism for fluid transport through rocks even when the abundance and interconnectivity of the reaction-induced porosity is low.

Radiation damage in multiphase ceramics

Four-phase ceramic composites containing 3mol% Y2O3 stabilized ZrO2 (3Y-TZP), Al2O3, MgAl2O4, and LaPO4 were synthesized as model materials representing inert matrix fuel with enhanced thermal conductivity and decreased radiation-induced microstructural damage with respect to single-phase UO2. This multi-phase concept, if successful, could be applied to design advanced nuclear fuels which could then be irradiated to higher burn-ups. 3Y-TZP in the composite represents a host (fuel) phase with the lowest thermal conductivity and Al2O3 is the high thermal conductivity phase. The role of MgAl2O4 and LaPO4 was to stabilize the structure under irradiation. The radiation response was evaluated by ion irradiation at 500°C with 10MeV Au ions and at 800°C with 92MeV Xe ions, to simulate damage due to primary knock-on atoms and fission fragments, respectively. Radiation damage and microstructural changes were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy and computational modeling. Al2O3, Y2O3 stabilized ZrO2 and MgAl2O4 phases exhibit high amorphization resistance and remain stable when irradiated with both Au and Xe ions. A monoclinic-to-tetragonal phase transformation, however, is promoted by Xe and Au ion irradiation in 3Y-TZP. The LaPO4 monazite phase appears to melt, dewet the other phases, and recrystallize under Au irradiation, but does not change under Xe irradiation.

Opportunities for Advanced Ceramics and Composites in the Nuclear Sector

Ceramics have played a crucial role in the development of fission based nuclear power, in glass & glass composite high level wasteforms, in composite cements to encapsulate intermediate level wastes (ILW) and also for oxide nuclear fuels based on UO2 and PuO2/UO2 mixed oxides. They are also used as porous filters with the ability to absorb radionuclides (RN) from air and liquids and are playing a key role in the cleanup at Fukushima. Non‐oxides also find current fission applications including in graphite moderators and B4C control rods. Ceramics will continue to be significant in the near‐term expansion of nuclear power via next‐step developments of fuels with inert matrices or based on thoria and in wasteforms using alternative composite cements or single or multiphase ceramics that can host Pu & other difficult RN. Longer term advances for Generation IV reactors, which will operate at higher temperatures & with higher fuel burn‐up require innovative fuel developments potentially via carbides & nitrides or composite fuel systems. Novel non‐thermal (cement‐like) and thermal techniques are currently being developed to treat some of the difficult legacy wastes. Non‐thermally derived wasteforms developed from geopolymers, composite cements, hydroceramics, and phosphate‐bonded ceramics and thermally derived wasteforms made by Hot Isostatic Pressing and fluidized bed steam reforming (FBSR) as well as vitrification techniques based on cold crucible melting (CCM), Joule‐heater in‐container melting and plasma melting (PM) are described. Future developments in waste treatment will be based on separation technologies for partitioning individual RN along with design & construction of RN‐containing ceramic targets for inducing transmutation reactions. Near demonstration actinide‐hosting ceramic wasteforms including multiphase Synroc systems are described. Opportunities also exist for ceramics in structural applications in Generation IV reactors such as composite SiC/SiC and C/C for fuel cladding and control rods and MAX phases and ultrahigh‐temperature ceramics (UHTCs) may find near core fuel coating and cladding applications. Uses of ceramics in fusion reactor systems will be both functional (ceramic superconductors in magnet systems for plasma control and in Li silicate breeder blankets in tokamaks) and structural including as sapphire diagnostic windows, graphite diverters, and plasma facing C and UHTCs. In all these cases, performance is limited by poorly understood radiation damage and interface controlled processes, which demands a combined modeling/experimental approach.

Effect of Heat on Hollow Multiphase Ceramic Microspheres Prepared by Self-Reactive Quenching Technology

The self-reactive quenching technology, which combines flame thermal spraying technology, self-propagating high-temperature synthesis (SHS), and rapid solidification, is a new method for preparation of hollow microspheres. Based on this, the effect of heat released by different exothermic systems on preparation of hollow ceramic microspheres was studied. The results show that for low-exothermic system Si-Sucrose-NH4Cl, the self-propagating reactions cannot occur, and the quenching products are Si microspheres with porous structure. For the moderate exothermic system Al–SiO2-Sucrose, the quenching products consist of some grains, which are hollow spherical or nearly spherical particles and irregular powders. Formation of Al2O3–Si indicates possible occurrence of SHS reactions. Meanwhile, for high-exothermic system Al–Cr2O3-Sucrose-Si-Epoxy Resin, the quenching products consist of internal hollow spherical grains and irregular-shaped porous particles; the phase composition mainly contains Al2O3, Cr3C2, Cr7C3, Cr3Si, and mullite, showing completeness of SHS reactions. The higher the adiabatic combustion temperature of the system is, the more heat it releases is higher, and the ceramic droplets form easilier.

Preparation and Oxidation of ZrB2/SiC/Zr2Al4C5 Multi-phase Ceramics with Spark Plasma Sintering

The ZrB2/SiC/Zr2Al4C5 multi-phase ceramics were fabricated by spark plasma sintering (SPS) at 1800 °C for 3 min under 20 MPa in an vacuum. Oxidation behavior of multi-phase ceramics were investigated using thermo gravimetric analysis (TGA) from 20 °C to 1500 °C and muffle furnace in stagnant air at 1200 °C. Samples were analyzed after oxidation by X-ray diffraction (XRD), scanning electron microscopy (SEM) along with energy dispersive spectroscopy (EDS) to determine the reaction products and to observe the microstructure. The results showed that the aluminium borate and mullite crystallize on the surface in the samples oxidized. The effect of Zr2Al4C5 content on the oxidation resistance of the ZrB2 ceramics were discussed respectively, and oxidation mechanism was also analysised.